The present invention relates to electric lamps, more specifically, to compact electrodeless fluorescent lamps operated at low and intermediate pressures and at frequencies above 20 kHz.
Electrodeless compact fluorescent lamps (ECFL) have been recently made available for indoor lighting. The advantage of such lamps is their long operating lifetime which is much longer than that of conventional compact fluorescent lamps employing heating filaments. The visible light is generated by an inductively coupled plasma that, in turn, is produced by a RF electromagnetic field generated in the lamp bulb by an induction coil.
A known compact electrodeless fluorescent lamp xe2x80x9cGenuraxe2x80x9d (General Electric Corp.) is operated at a RF frequency of 2.65 MHz and utilizes an induction coil with a ferrite core inserted in a reentrant cavity formed in a transparent bulb container. Genura is marketed as a replacement for an R30 incandescent lamp and is indicated to have 1,100 lumen light output at 23 W of RF power with an operating lifetime of 15,000 hrs. The drawback of the Genura lamp is its high initial cost, and relatively large diameter (80 mm) that is larger than that of a 100-W incandescent lamp (60 mm) having 1500 lumen light output. The latter characteristic imposes some restrictions on the conditions of lamp usage. In addition, the lamp employs an internal reflector and so can be used only in recessed lamp holding fixtures for downward lighting applications.
The high initial cost of the Genura lamp is due to the high cost of the driver electronic circuitry because of being operated at a frequency of 2.65 MHz, and which must include a special circuit to prevent electromagnetic interference (EMI). Thus, the use of a lower frequency of approximately 100 kHz is desired to reduce the initial lamp cost.
Also, a compact electrodeless fluorescent lamp is desired that is smaller than the Genura lamp (i.e. made with a 60 mm diameter equivalent to that of a A25 bulb) and that can be used in regular fixtures for both upward lighting and downward lighting applications.
In a copending U.S. Patent Application entitled xe2x80x9cHigh Frequency Electrodeless Compact Fluorescent Lampxe2x80x9d having Ser. No. 09/435,960 by Chandler et al. and assigned to the same assignee as the present invention, a compact electrodeless fluorescent lamp is disclosed that is operated at relatively xe2x80x9clowxe2x80x9d frequencies from 50 kHz to 500 kHz. The lamp utilizes a ferrite core and a thin ferrite disk attached to the core bottom both made from MnZn material. A multiple insulated strand wire (Litz wire) is used for the induction coil that is wound in two layers around the ferrite core.
Two types of cooling structures that remove the heat generated during operation from the cavity and the ferrite core are described in that application. The first structure comprises a copper tube inside the ferrite core that protrudes along the lamp base down to the Edison socket cup and is welded to a copper cylinder in the Edison socket cup. Such an arrangement provides for the transmission of heat from the cavity/ferrite core to the Edison socket cup and then to the lamp holding fixture. However, this approach has two disadvantages. In many applications, the Edison socket cup does not have a good thermal contact with the fixture, and thus the resulting relatively poor thermal conduction leads to an increase of the ferrite core material operating temperature to values higher than its Curie point. The second disadvantage is the position of the metal (or ceramic) cooling tube in the base center, along its axis, that makes it difficult to place the driver electronic circuitry inside the base.
The other structure taught in this application comprises a metal tube inside the ferrite core and a ceramic structure that is thermally connected to the tube. The ceramic structure has a shape of a xe2x80x9cskirtxe2x80x9d and transfers the heat from the cavity and the core to the atmosphere via convection.
Both of these types of cooling structures provide acceptable ferrite core temperatures during operation, that is, temperatures lower than the ferrite material Curie point of 220xc2x0 C., and sufficiently low temperature inside the lamp base ( less than 100xc2x0 C.), when the lamp is operated without a lamp holding fixture at an ambient temperature of 25xc2x0 C. However, neither of these arrangements may always provide the desired operating temperatures when the lamp is inserted in a lamp holding fixture that has the effect of increasing the effective lamp xe2x80x9cambientxe2x80x9d temperature up to 50-60xc2x0 C. Therefore, a more efficient cooling structure is desired for reliable operation of such lamps in a holding fixture.
Also, the use of ceramic (alumina) material structure is rather costly so that the initial cost of the lamp may be unacceptably high. The use of materials less expensive than alumina but with the same (or higher) thermal conductivity is desirable to reduce the initial cost of the lamp cooling structure and, hence, of the whole lamp system.
The present invention comprises a compact electrodeless fluorescent lamp that includes a transparent envelope containing a fill of inert gas along with a vaporizable metal such as mercury. An induction coil, such as one formed by Litz wire, is operated by a driver circuit, and is positioned inside of a reentrant cavity in the envelope with an adjacent permeable magnetic field manipulation structure having a shunting surface ending at a shunting surface periphery. The magnetic field manipulation structure may comprise a toroid with a disk-like base, and may be formed of a ferrite material. A thermally and electrically conductive primary cooling structure is positioned adjacent the magnetic field manipulation structure to extend within the shunting surface periphery while being separated from the induction coil thereby. The primary cooling structure may comprise a thermally conductive tube, such as a metal tube, for instance copper, placed inside of the cavity extending so as to extend with in the toroid, and may have a finned dissipater provided therewith.
A further component cooling structure is provided to at least partially enclose the driver circuit connected to the induction coil. This component cooling structure is separate from the primary cooling structure, and may cool at least an electrolytic capacitor in the driver circuit.